1. Field of the Invention
The present invention relates to an image forming apparatus, an image processing apparatus, and a control method for them and, more particularly, to an image forming apparatus, an image processing apparatus, and a control method for them, which perform image formation at a lower tone level than that of an input image.
2. Description of the Related Art
A general example of an image output apparatus for a word processor, personal computer, facsimile apparatus, and the like is a printing apparatus which prints information such as desired characters and images on a sheet-like print medium such as a paper sheet or film. Such printing apparatuses use various printing methods. Among them, methods of forming an image on a print medium by making ink adhere to the print medium have been widely implemented into practice. As a typical example of such methods, the inkjet printing method has been known.
In order to increase the printing speed and improve the image quality, a printing apparatus using the inkjet printing method includes a nozzle group which is an integral array of a plurality of ink orifices (nozzles) capable of discharging ink having the same color and density. To further improve the image quality, some apparatuses include a nozzle group capable of discharging inks having the same color but different densities or a nozzle group of capable of discharging ink having the same color and density while changing the discharge amount of ink in some steps.
In such a printing apparatus, the quality of the image to be printed greatly depends on the performance of a printhead as a discrete component. For example, a slight error in the process of manufacturing a printhead such as variations in the shape of the orifices of a printhead, discharge heaters, or piezoelectric elements greatly affects the discharge amount and direction of ink to be discharged. That is, such a slight error is a factor that causes density unevenness in the image to be formed and degrades the image quality.
As a measure for suppressing such density unevenness, the multi-pass printing method is known (see, for example, Japanese Patent Laid-Open No. 2002-096455). According to this technique, combining image processing and printing control makes it possible to form an image at high speed while suppressing deterioration in image quality due to white stripes or density unevenness.
The multi-pass printing method will be described in detail below with reference to FIG. 51.
Referring to FIG. 51, reference numeral 5101 denotes a multi-head. For simplicity, assume that the multi-head 5101 comprises eight nozzles 5102. Reference numeral 5103 denotes an ink droplet discharged by the nozzle 5102. In general, when the same main scanning print area on a predetermined print medium is to be completed in one scan, it is ideal to discharge inks at a uniform discharge amount in a uniform direction, as shown in FIG. 51.
In practice, however, each nozzle varies. If, therefore, printing is done in one scan, ink drops discharged from the respective nozzles vary in size and direction. As a result, blank portions periodically exist in the head main scanning direction, or dots are excessively superimposed in some portions. A set of dots landed in this state is perceived as density unevenness in the nozzle array direction.
For this reason, the multi-pass printing method performs main scanning a plurality of number of times (three times in this case) using a multi-head 5202, as shown in FIG. 52. Referring to FIG. 52, a printing scan area comprising, as a unit, four pixels which are half in number of eight pixels in the vertical direction is completed by two printing scans (passes). In this case, the eight nozzles in the multi-head 5201 are divided into a group of four upper nozzles (upper nozzle group) and a group of four lower nozzles (lower nozzle group). The dot printed by one nozzle in one scan corresponds to the data obtained by thinning out image data to about ½ in accordance with a predetermined image data arrangement. Embedding about half the remaining number of dots in the image formed in advance at the time of the second scan will complete a four-pixel unit area.
In the multi-pass printing method, the first and second scans performed in accordance with a predetermined arrangement compensate for each other. As an image data arrangement (thinning-out mask pattern) used for this purpose, an arrangement in which pixels are vertically and horizontally staggered one by one as shown in FIG. 53 is generally used. In a unit print area (a four-pixel unit area in this case), therefore, printing is completed by the first scan of printing a staggered pattern and the second scan of printing an inverse staggered pattern. The upper, middle, and lower parts of FIG. 53 show how printing in the same area is completed using the staggered and inverse staggered patterns described above. First of all, in a predetermined area on a print medium, as shown in the upper part of FIG. 53, the first scan prints a staggered pattern (black circles) using four lower nozzles. As shown in the middle part of FIG. 53, the second scan prints an inverse staggered pattern (white circles) in the area using all the eight nozzles while feeding the sheet by four pixels. In addition, as shown in the lower part of FIG. 53, the third scan prints a staggered pattern in the area again using the four upper nozzles while feeding the sheet by four pixels.
Executing this multi-pass printing method, even in the use of a multi-head with variations like those shown in FIG. 52, will halve the influence of the variations on a print area, thereby reducing the density unevenness in the image to be formed.
Even if, however, such multi-pass printing is performed, the problem of density unevenness may not be easily solved depending on the printing ratio of each printing scan. For this reason, there has been proposed a technique of changing the pitch of the respective areas by randomly changing the subscanning direction feeding amount (sheet feeding amount) between the respective printing scans in the execution of multi-pass printing, thereby reducing density unevenness (see, for example, Japanese Patent Laid-Open No. 7-52465).
There has also been proposed a method of reducing image quality deterioration in the leading edge and trailing edge areas of a print medium in the feeding direction by changing the number of nozzles of a printhead which are to be used in one printing scan in the multi-pass printing method (see, for example, Japanese Patent Laid-Open No. 11-245388). According to this technique, since the positional accuracy of print medium feeding deteriorates in the leading edge and trailing edge areas of a print medium in the feeding direction, deterioration in printed image quality due to positional accuracy deterioration is made less noticeable by decreasing the feeding amount and the number of nozzles to be used in such areas.
The multi-pass printing method of randomly changing the sheet feeding amount can reduce density unevenness which periodically occurs in the subscanning direction. However, according to this method, since an image is formed in each main scan, if a sheet feeding error occurs, the graininess of an image deteriorates, and hence the image quality of the output image deteriorates.
The manner of how graininess deteriorates due to this sheet feeding error will be described below with reference to FIGS. 54 and 55. FIGS. 54 and 55 each show an example of how two-pass printing is performed using a staggered mask pattern like that shown in FIG. 53. FIG. 54 shows a print example without any sheet feeding error in a print medium. FIG. 55 shows a print example with a sheet feeding error.
As shown in FIG. 54, if there is no sheet feeding error, dots are accurately printed at the positions designated by an input binary halftone image. Assume that in this case, no nozzle discharge variation has occurred. If, however, as shown in FIG. 55, the feeding of the print medium shifts in the arrow direction before the second pass printing, the image formed on the print medium deteriorates and becomes grainy. This is because in the multi-pass printing method using a mask pattern, even if the dispersibility of the image to be finally output is high, the dispersibility of dots to be printed in each pass is low, as shown in FIG. 55.
In addition, since the sheet feeding error amount varies for each sheet feeding operation, the following problem arises. For example, even in an area with the same input data value, if the state shown in FIG. 54 without any sheet feeding error appears adjacent to the state shown in FIG. 55 with the sheet feeding error, density unevenness with a longer period than that caused by nozzle variations occurs.
The following problem arises in the above method of reducing image quality deterioration in the leading and trailing edge areas of a recording member in the feeding direction by changing the number of nozzles of the printhead which are to be used in one printing scan. This method allows setting only a sheet feeding amount corresponding to an integer multiple of the minimum unit of sheet feeding amount (e.g., an amount corresponding to a predetermined number of nozzles). This may make it impossible to efficiently use a limited number of nozzles.